The present invention relates to a superheated steam generator comprising an outer ferrule having at its opposite ends a chamber for the intake of water to be vapourized and superheated and a steam outflow chamber and a group of tubes connecting said chambers in which circulates the water to be vaporized and superheated by heat exchange with a hotter fluid circulating in counterflow around the tubes within the outer ferrule.
In such a steam generator, the wall of the steam outlet chamber may comprise:
(a) a first part constituted by a thick tube plate with a concave surface on the side of the chamber and an outer annular area, the whole of the first part being obtained by forging a single cylindrical blank, PA1 (b) a second hemispherical part constituted by a stamped or forged end connected to the first part by a circumferential weld located in a plane perpendicular to the axis of the steam generator.
Very high temperature and very high pressure superheated steam passes through such steam outlet chambers. They have a steam outlet mounting and one or more inspection holes. The tube plate and bottom, which form their walls, must be very thick. Therefore very high thermal stresses occur during transient operating conditions, on starting, stopping or operation change. These stresses appear in the walls of the chamber due to the delay with which the outer wall thereof follows the temperature variations of the inner wall. They also occur on the periphery of the tube plate, due to the thermal inertia difference between the perforated area of the plate and the unperforated peripheral area. They also occur in the connection area between the chamber enclosure and the steam discharge mounting as a result of their thickness differences, as well as in the connection area of the chamber envelope with the outer ferrule of the exchanger. Particularly in the case of a superheated steam discharge chamber of a heat exchanger between the water to be vaporized and superheated and liquid sodium in an installation for the generation of electric power from a fast breeder reactor, the superheated steam leaves at a pressure of about 200 bars and a temperature of about 500.degree. C. As a result of this pressure and temperature, it is necessary to make the tube plate and chamber bottom very thick. The operating temperature, which is much higher than ambient temperature produces important temperature variations. These give rise to high thermal stresses during transient operating states, so that special precautions must be taken in the operation of such exchangers.